Rule 11-605 Judge’s competency as a witness.

The judge running your trial can't also take the witness stand and testify in that same case. This keeps the judge's role separate from the role of a witness, so the person deciding the case isn't also giving testimony that gets weighed as evidence.

If this happens, a party doesn't have to speak up and object at the time to protect their right to challenge it later. This is different from most trial mistakes, where you must object right away or you lose the right to complain about it on appeal. Here, the problem is considered so serious that the objection is automatically preserved, even if no one says anything during the trial.